10 GEOLOGICAL NOTES
or more or less peaty clay or mud, with shell marl, intervene
between them. And it seems clear to me that the gravel has
always been brought down in the channel of the stream ; and
that the surface loam is the mud which has been deposited on
the surface during floods and very high tides. Where, between
the gravel and the loam, there is a considerable thickness of
peat, the plants, the remains of which compose it, having grown
in situ, there shallow backwaters must have existed in which
the plants grew. On the other hand, the irregular masses of
vegetable matter mixed more or less with gravel and sand have
evidently been brought down a channel, mainly in time of flood.
And the little land and fresh-water mollusca, composing the shell
marl, flourished either in backwaters or in shallow parts of a
main stream where sand or mud were being deposited close to
one bank, while that on the opposite side was suffering from
erosion. The steady continuance of a state of things of this
kind, al any one spot, for many years, would bring into existence
a band of shell marl having a breadth corresponding to the
lateral deflection of the channel of the stream.
Fig. 7. New Reservoirs. Section of Minor Channel in Southern Reservoir
(1901). 1. Gravel: 2. Mud; 3. Peat, with many Plant-stems; 4. Sand with
Shells in channel; 5. Surface Loam. Height of Section, 10 to 11 ft. ;
length, about 18 yards.
The section across the old channel, shown in Fig. 5, illus-
trates my remark that the constant constituents of a section are
the gravel towards the base and the loam at the surface. Taking
the height of the section as 10ft., then the changes are all in the
5ft., or thereabouts, of beds in the middle part; the gravel
towards the base and the loam at the surface remain unaltered
throughout. This is especially noteworthy in the case of the
loam, and clearly indicates that it is a deposit resulting from a
general cause which, like a flood, would affect a whole river
valley. It would consequently not be affected by the local
character of the beds on which it rested, and would tend to
equalise the surface levels, as we know it does.